Allylboronate transmetallation

If the presence of sensitive functional groups poses problems of chemose-lectivity in the use of hard allylic metal reagents, allylboronate derivatives also can be accessed by a milder transmetalation of allylic tin species with boron halides.This approach has been used by Corey in the synthesis of chiral bis(sulfonamido)boron reagents such as the medially 1 reagent 15 (Eq. 19) (see section Chiral Boronate Derivatives ). ... 

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. 

Activation of allyl alcohols is treated in Chapter 4.1. Concerning nucleophilic allylation, it has been shown that ciimamyl alcohol (324) reacts smoothly with benzaldehyde in the presence of Pd catalyst and Et3B as a stoichiometric reagent in THE at room temperature to afford homoallylic alcohol 326 [119]. In this reaction, nucleophilic allylboron 325 is generated by transmetallation of 7r-allylpalladium with Et3B and reacts with benzaldehyde. 

Recently, Shibasaki and co-workers reported a copperreaction using a chiral diphosphine ligand, DuPHOS, with an added lanthanide salt (118]. This new allylation system provides good levels of enantioselectivity in additions of the simple allylboronate 2 to either aromatic or aliphatic ketones that present a large difference of steric bulk on both sides of the carbonyl (Equation 43). Based on NMR experiments and on the lack of diastereoselectivity in crotylation examples, the suggested mechanism of this allylation involves transmetallation of the boron to an allylcopper species. 

Transmetallation reactions between boron trihalides and organometallic reagents containing Si, Hg, Sn," Zr, and Cu" are practical and valuable routes to arylboron halides and allylboron halides since hydroboration reactions using haloboranes are not possible for these moieties. 

Since palladium complexes have proven to be efficient in activating diboron compounds throughout transmetalation, a wide range of applications have been considered in the last decade, such as palladium-catalyzed transformation of aUyhc alcohols to allylboronates, borylation of allylic hahdes or aUyhc acetates, and the P-boration of a,P-unsaturated carbonyl substrates.Interestingly, both palladium and nickel showed to be similarly efficient to activate B2pin2 and catalyze the addition to unsaturated substrates. Oshima postulated that Ni(0) species react with substrate a,P-unsaturated esters and amides to generate the r -coordinated complex, which activates the B2pin2 to favor the formation of T -coordinated... 

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